1. Field of the Invention
The present invention relates to a compound for detecting a phosphorylated peptide or protein that recognizes a phosphate group at multiple sites. Furthermore, the present invention relates to a method for detecting a multisite phosphorylated peptide or protein in a sample using the compound. In particular, the present invention relates to a compound that specifically recognizes an excessively phosphorylated tau protein, a method for detecting a phosphorylated tau protein in a sample using the compound, and a method for detecting Alzheimer's disease, an imaging method, a method for diagnosing Alzheimer's disease and a reagent kit using the compound.
2. Description of the Related Art
Proteins in the body are subjected to various biochemical modifications and change their higher-order structures, functions and activities to regulate biological function. Protein phosphorylation, one of protein modifications, is a posttranslational modification catalyzed by protein phosphorylating enzymes (protein kinases) using ATP as a phosphate group donor. Protein phosphorylation is closely associated with various cellular activities such as carbohydrate metabolism, cell growth/division, intracellular signal transduction, and enzyme activity regulation. Protein phosphorylation is an important process for regulating protein activity, and it is estimated that approx. 30% of proteins are subjected to phosphorylation in some manner in eukaryotes (for example, refer to Matthias Mann et al., Trends Biotechnol. [2002] 20, 261-268). Phosphorylation states in the body are strictly regulated by protein phosphorylating enzymes (kinases) and dephosphorylating enzymes (phosphatases) to maintain normal physiological functions. It has been reported that abnormal regulation of phosphorylation causes various diseases including cancer. Drugs for regulating phosphorylation have been searched, and several kinase inhibitors have been clinically applied.
Alzheimer's disease is known as one of diseases characterized by abnormally phosphorylated proteins as pathological changes. Alzheimer's disease is one of diseases difficult to treat, and studies aiming for correct early diagnoses and early treatment have been conducted. As a pathological characteristic of Alzheimer's disease, senile plaques and neurofibrillary tangles are confirmed in the patient's brain. The neurofibrillary tangle is accumulation of a double-helical fibrous protein called paired helical filament (PHF) in the nerve cell. One of its components is a tau protein, one of microtubule-associated proteins specific to the brain (for example, refer to Yasuo Ihara et al., Journal of Biochemistry [Tokyo], [1986] 99, 1807-1810 and Inge Grundke-Iqbal et al., Proc. Natl. Acad. Sci. USA 83, 4913-4917). It has been found that the tau proteins incorporated in PHFs in the brain affected by Alzheimer's disease are abnormally phosphorylated as compared with normal tau proteins, and their phosphorylation sites have been identified (for example, refer to Japanese Patent Application Laid-Open No. 6-239893). In addition to Alzheimer's disease, diseases that present accumulation of tau proteins in the brain as the principal sign (tauopathies) include Pick disease, progressive supranuclear palsy, and frontotemporal dementia, and all these diseases are closely associated with phosphorylated tau proteins. Therefore, detection, in vivo or in vitro, of a phosphorylated tau protein as a marker is one of excellent methods for diagnosing diseases characterized by accumulation of phosphorylated tau proteins, in particular, Alzheimer's disease.
As described above, the consensus is that monitoring of phosphorylation of proteins closely associated with a disease is very important and effective in the fields of biomedical studies, laboratory tests, and in-vivo image diagnoses. To achieve this monitoring with high sensitivity and high precision, it is necessary to develop compound molecules for monitoring phosphorylated proteins specifically with high sensitivity and techniques for detecting these proteins.